Japanese Patent Laid-Open No. 2001-227399 discloses technology (hereunder, referred to as a “related art”) relating to switch control of a combustion mode in an internal combustion engine capable of switching the combustion mode of the internal combustion engine from a stratified charge combustion mode to a homogeneous combustion mode, or from the homogeneous combustion mode to the stratified charge combustion mode. Before and after switch of the combustion mode, control parameters such as a degree of throttle opening and an EGR amount are adjusted to keep a stable combustion state, and when these control parameters change, a torque fluctuation occurs. According to the above described related art, when the control parameters to be the cause of a torque fluctuation are adjusted before and after switch of the combustion mode, the torque fluctuation amount by the adjustment amounts is converted into the change amounts of control parameters that do not relate to the adjustment, and the control parameters are changed by the change amounts.
Incidentally, there has been conventionally known an internal combustion engine capable of performing switching control of an air-fuel ratio such as switching from the operation by a theoretical air-fuel ratio to the operation by an air-fuel ratio leaner than the theoretical air-fuel ratio, or switching in the opposite way. The above described related art discloses the above described art also being able to be applied at the time of switch of a lean operation and a stoichiometric/rich operation in a lean burn engine like this.
However, when the above described related art is applied to a lean burn engine equipped with a turbocharger, a torque fluctuation at the time of switch of an air fuel ratio cannot be always restrained. That is to say, when the air-fuel ratio is switched from a theoretical air-fuel ratio to an air-fuel ratio that is leaner than the theoretical air-fuel ratio at the time of deceleration of the engine equipped with a turbocharger, for example, the air fuel ratio to be a target is switched from an amount corresponding to the theoretical air-fuel ratio to an amount corresponding to a lean air-fuel ratio. However, even when the air amount to be the target is switched, an actual air amount does not immediately track the air amount to be the target. This is because a response delay due to a turbo lag occurs in the engine equipped with a turbocharger, in addition to a response delay of a throttle and the like which are actuators that regulate the air amount. Since in a time period in which the actual air amount is smaller than the target air amount, the actual torque is larger than a requested torque, the requested torque is realized by retardation control of the ignition timing, and the torque level difference is eliminated. However, when the time period until the target air amount is realized becomes long due to the influence of the above described turbo lag, the retardation control of the ignition timing is restricted by a constraint such as an upper limit of the temperature of the exhaust system, as a result of which, a torque level difference is caused.